1. Field of the Invention
The present invention relates to an apparatus for detecting breakage of a looper thread by monitoring vibration generating in looper thread along with driving of a sewing machine when forming a double-chain stitch in a cloth by the sewing machine.
2. Description of the Prior Art
In an ordinary conventional sewing machine comprising a needle moving up and down to penetrate through a needle hole of a needle plate, and a looper reciprocating laterally across the leading end of this needle, for forming a double-chain stitch by collaboration of the needle and looper, usually, a thread-controlling cam varying in the extent of projection from the thread take-up stand according to the rotation of the rotary shaft such as main shaft is disposed between two thread guides having a thread eyelet for inserting the looper thread, and the looper thread slides on the peripheral surface of this thread-controlling cam, so that it is designed to control the thread take-up extent.
In the sewing machine having such constitution, however, if the thread is broken during sewing operation, or if the thread is loose when driving the sewing machine again right after cutting off the thread before sewing operation or after sewing operation, the looper thread may wind around the rotary shaft of the thread-controlling cam. If driving of the sewing machine is continued without noticing the winding of looper thread around the rotary shaft of the thread-controlling cam, the looper thread is drawn out from the supply source side, and the winding amount on the rotary shaft increases progressively, and it takes much time and labor for removing the thread winded around the rotary shaft, which may cause to lower the sewing operation efficiency extremely. In particular, in the narrow tube bed sewing machine for sewing a tubular cloth, since the space in the narrow tube bed is small, it is very difficult to remove the thread winded around the rotary shaft.